diff --git a/dfssc/sign/create.go b/dfssc/sign/create.go
index 36f86e27b0d2176fc6fe660519743afc1f4cdf34..ada6a6c4dfbde3091a1cbb027fc5e1db7886b473 100644
--- a/dfssc/sign/create.go
+++ b/dfssc/sign/create.go
@@ -2,7 +2,6 @@ package sign
 
 import (
 	"crypto/sha512"
-	"fmt"
 	"io/ioutil"
 	"path/filepath"
 	"time"
@@ -26,7 +25,7 @@ type CreateManager struct {
 	filepath   string
 	comment    string
 	signers    []string
-	hash       string
+	hash       []byte
 	filename   string
 }
 
@@ -64,7 +63,7 @@ func (m *CreateManager) computeFile() error {
 	}
 
 	hash := sha512.Sum512(data)
-	m.hash = fmt.Sprintf("%x", hash)
+	m.hash = hash[:]
 	m.filename = filepath.Base(m.filepath)
 
 	return nil
diff --git a/dfssc/sign/create_test.go b/dfssc/sign/create_test.go
index e12631fe2437397046bc3aa69b66eb80a8088f8f..bce905ef6644fbe1ff6bf4e788c5c9d63e771f66 100644
--- a/dfssc/sign/create_test.go
+++ b/dfssc/sign/create_test.go
@@ -1,6 +1,7 @@
 package sign
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -55,7 +56,7 @@ func TestComputeFile(t *testing.T) {
 	m := &CreateManager{filepath: fcontract}
 	err := m.computeFile()
 	assert.Equal(t, nil, err)
-	assert.Equal(t, "37fd29decfb2d689439478b1f64b60441534c1e373a7023676c94ac6772639edab46f80139d167a2741f159e62b3064eca58bb331d32cd10770f29064af2a9de", m.hash)
+	assert.Equal(t, "37fd29decfb2d689439478b1f64b60441534c1e373a7023676c94ac6772639edab46f80139d167a2741f159e62b3064eca58bb331d32cd10770f29064af2a9de", fmt.Sprintf("%x", m.hash))
 	assert.Equal(t, "contract.txt", m.filename)
 
 }
diff --git a/dfssp/api/gRPCServices.pb.go b/dfssp/api/gRPCServices.pb.go
index a97a93cb0d56603242adc0f20eba85a4dc07b82e..9991ded69708304ef0b1b0d0719f4c1f8cc2eb66 100644
--- a/dfssp/api/gRPCServices.pb.go
+++ b/dfssp/api/gRPCServices.pb.go
@@ -132,7 +132,7 @@ func (*Empty) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{4} }
 // PostContractRequest message contains the contract as SHA-512 hash, its filename,
 // the list of signers as an array of strings, and a comment
 type PostContractRequest struct {
-	Hash     string   `protobuf:"bytes,1,opt,name=hash" json:"hash,omitempty"`
+	Hash     []byte   `protobuf:"bytes,1,opt,name=hash,proto3" json:"hash,omitempty"`
 	Filename string   `protobuf:"bytes,2,opt,name=filename" json:"filename,omitempty"`
 	Signer   []string `protobuf:"bytes,3,rep,name=signer" json:"signer,omitempty"`
 	Comment  string   `protobuf:"bytes,4,opt,name=comment" json:"comment,omitempty"`
@@ -374,36 +374,36 @@ var _Platform_serviceDesc = grpc.ServiceDesc{
 }
 
 var fileDescriptor0 = []byte{
-	// 489 bytes of a gzipped FileDescriptorProto
+	// 490 bytes of a gzipped FileDescriptorProto
 	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0x94, 0x53, 0x5d, 0x8f, 0xd2, 0x40,
-	0x14, 0xdd, 0x42, 0x97, 0x8f, 0xbb, 0xbb, 0xd8, 0x0c, 0x68, 0x2a, 0x0f, 0x66, 0x33, 0x2f, 0x1a,
-	0x1f, 0xd0, 0xa0, 0x31, 0xd1, 0xf8, 0x52, 0x2b, 0x59, 0xd7, 0x07, 0x42, 0xa6, 0x5b, 0x7d, 0xae,
-	0xe5, 0x2e, 0x34, 0xd2, 0x0e, 0x4e, 0x07, 0x0d, 0x7f, 0xc6, 0x3f, 0xe4, 0x8f, 0xd2, 0xce, 0xb4,
-	0x43, 0xa8, 0x36, 0x26, 0xcb, 0x03, 0xcc, 0xb9, 0x1f, 0xe7, 0xde, 0x39, 0x67, 0x00, 0xb2, 0x62,
-	0x0b, 0x3f, 0x40, 0xf1, 0x3d, 0x89, 0x31, 0x9f, 0x6c, 0x05, 0x97, 0x9c, 0xb4, 0xa3, 0x6d, 0x42,
-	0x3d, 0xb8, 0xc7, 0x70, 0x95, 0xe4, 0x12, 0x05, 0xc3, 0x6f, 0x3b, 0xcc, 0x25, 0x19, 0xc1, 0x29,
-	0xa6, 0x51, 0xb2, 0x71, 0xad, 0x4b, 0xeb, 0x49, 0x9f, 0x95, 0x80, 0xb8, 0xd0, 0x15, 0x65, 0x81,
-	0xdb, 0xd2, 0x71, 0x03, 0xe9, 0x4f, 0x0b, 0xfa, 0x33, 0x21, 0xb8, 0xf0, 0xf9, 0x12, 0xc9, 0x63,
-	0xb0, 0xe3, 0xe2, 0x57, 0x37, 0x0f, 0xa6, 0xc3, 0x49, 0x31, 0x64, 0x72, 0xc8, 0x4e, 0xd4, 0x17,
-	0xd3, 0x05, 0x8a, 0x30, 0xc5, 0x3c, 0x8f, 0x56, 0x68, 0x08, 0x2b, 0x48, 0xe7, 0x60, 0x6b, 0xaa,
-	0x33, 0xe8, 0x06, 0xa1, 0xef, 0xcf, 0x82, 0xc0, 0x39, 0x21, 0x00, 0x9d, 0xeb, 0xf9, 0x27, 0x8f,
-	0x5d, 0x39, 0x96, 0x4a, 0xbc, 0xf3, 0xde, 0x7b, 0xe1, 0xcd, 0x07, 0xa7, 0xa5, 0xc0, 0x67, 0x8f,
-	0xcd, 0xaf, 0xe7, 0x57, 0x4e, 0x9b, 0x0c, 0x55, 0xd5, 0xcd, 0x8c, 0x31, 0xe7, 0xb7, 0xf9, 0x58,
-	0xf4, 0x35, 0x9c, 0x79, 0x3b, 0xb9, 0xfe, 0xff, 0xfd, 0x8a, 0xa8, 0xe4, 0x5f, 0x31, 0xab, 0x96,
-	0x29, 0x01, 0x7d, 0x0e, 0x03, 0x23, 0x0f, 0x2e, 0xc3, 0x1c, 0x05, 0x79, 0x04, 0x10, 0x6f, 0x12,
-	0xcc, 0xa4, 0x8f, 0x42, 0x56, 0x14, 0x47, 0x11, 0xda, 0x85, 0xd3, 0x59, 0xba, 0x95, 0x7b, 0xfa,
-	0x03, 0x86, 0x0b, 0x9e, 0x4b, 0x9f, 0x67, 0x52, 0x44, 0xb1, 0x34, 0xd3, 0x09, 0xd8, 0xeb, 0x28,
-	0x5f, 0x57, 0x9d, 0xfa, 0x4c, 0xc6, 0xd0, 0xbb, 0x4d, 0x36, 0x98, 0x45, 0xa9, 0xd1, 0xe2, 0x80,
-	0xc9, 0x03, 0xe8, 0xe4, 0xc9, 0x2a, 0x43, 0xe1, 0xb6, 0x2f, 0xdb, 0x45, 0xa6, 0x42, 0x4a, 0xbe,
-	0x98, 0xa7, 0x69, 0x31, 0xd6, 0xb5, 0x4b, 0xf9, 0x2a, 0x58, 0xc8, 0x37, 0xfa, 0xc8, 0x93, 0x2c,
-	0x28, 0xea, 0x22, 0xb9, 0x13, 0x68, 0x26, 0x53, 0x38, 0x8f, 0xab, 0x65, 0xc2, 0x5d, 0xb2, 0xac,
-	0x36, 0xa8, 0xc5, 0xd4, 0x76, 0x5b, 0x2e, 0x4a, 0x8b, 0x2f, 0x98, 0x3e, 0xd3, 0x57, 0xe0, 0x30,
-	0x8c, 0x96, 0x7b, 0x45, 0x78, 0x07, 0xae, 0xe9, 0xaf, 0x16, 0xf4, 0x16, 0x9b, 0x48, 0xde, 0x72,
-	0x91, 0x92, 0x29, 0xf4, 0x8c, 0x90, 0x64, 0xa4, 0x1f, 0xc5, 0x5f, 0xcf, 0x6e, 0x3c, 0xa8, 0x3f,
-	0x15, 0x7a, 0x42, 0x9e, 0x81, 0xad, 0x7c, 0x23, 0x8e, 0xce, 0x1c, 0x59, 0x38, 0x1e, 0xd6, 0x18,
-	0x4a, 0x67, 0x8a, 0x86, 0xa7, 0x00, 0x61, 0x26, 0xcc, 0x18, 0x28, 0x09, 0x95, 0x19, 0x0d, 0xe4,
-	0x6f, 0xe0, 0xfc, 0xd8, 0x1e, 0xe2, 0xea, 0x8a, 0x06, 0xc7, 0x1a, 0x7a, 0xdf, 0xc2, 0x45, 0x4d,
-	0x61, 0xf2, 0x50, 0x97, 0x34, 0xa9, 0xde, 0xd0, 0xfd, 0x12, 0xfa, 0x07, 0x3d, 0xc9, 0xfd, 0xea,
-	0x26, 0x75, 0x7d, 0xff, 0xed, 0xfa, 0xd2, 0xd1, 0x7f, 0xda, 0x17, 0x7f, 0x02, 0x00, 0x00, 0xff,
-	0xff, 0x59, 0x0a, 0x30, 0x18, 0xca, 0x03, 0x00, 0x00,
+	0x14, 0xdd, 0x42, 0x97, 0x8f, 0xbb, 0x2c, 0x36, 0x03, 0x9a, 0xca, 0x83, 0xd9, 0xcc, 0x8b, 0xc6,
+	0x07, 0x34, 0x68, 0x4c, 0x34, 0xbe, 0xd4, 0x4a, 0xd6, 0xf5, 0x81, 0x90, 0xe9, 0x56, 0x9f, 0x6b,
+	0xb9, 0x0b, 0x8d, 0xb4, 0x83, 0xd3, 0x41, 0xb3, 0x7f, 0xc6, 0x3f, 0xe4, 0x8f, 0xd2, 0xce, 0xb4,
+	0x43, 0xa8, 0x36, 0x26, 0xcb, 0x03, 0xcc, 0xb9, 0x1f, 0xe7, 0xde, 0x39, 0x67, 0x00, 0xb2, 0x66,
+	0x4b, 0x3f, 0x40, 0xf1, 0x3d, 0x89, 0x31, 0x9f, 0xee, 0x04, 0x97, 0x9c, 0xb4, 0xa3, 0x5d, 0x42,
+	0x3d, 0xb8, 0xc7, 0x70, 0x9d, 0xe4, 0x12, 0x05, 0xc3, 0x6f, 0x7b, 0xcc, 0x25, 0x19, 0xc3, 0x29,
+	0xa6, 0x51, 0xb2, 0x75, 0xad, 0x0b, 0xeb, 0x49, 0x9f, 0x95, 0x80, 0xb8, 0xd0, 0x15, 0x65, 0x81,
+	0xdb, 0xd2, 0x71, 0x03, 0xe9, 0x4f, 0x0b, 0xfa, 0x73, 0x21, 0xb8, 0xf0, 0xf9, 0x0a, 0xc9, 0x63,
+	0xb0, 0xe3, 0xe2, 0x57, 0x37, 0x0f, 0x67, 0xa3, 0x69, 0x31, 0x64, 0x7a, 0xc8, 0x4e, 0xd5, 0x17,
+	0xd3, 0x05, 0x8a, 0x30, 0xc5, 0x3c, 0x8f, 0xd6, 0x68, 0x08, 0x2b, 0x48, 0x17, 0x60, 0x6b, 0xaa,
+	0x33, 0xe8, 0x06, 0xa1, 0xef, 0xcf, 0x83, 0xc0, 0x39, 0x21, 0x00, 0x9d, 0xab, 0xc5, 0x27, 0x8f,
+	0x5d, 0x3a, 0x96, 0x4a, 0xbc, 0xf3, 0xde, 0x7b, 0xe1, 0xf5, 0x07, 0xa7, 0xa5, 0xc0, 0x67, 0x8f,
+	0x2d, 0xae, 0x16, 0x97, 0x4e, 0x9b, 0x8c, 0x54, 0xd5, 0xf5, 0x9c, 0x31, 0xe7, 0xb7, 0xf9, 0x58,
+	0xf4, 0x35, 0x9c, 0x79, 0x7b, 0xb9, 0xf9, 0xff, 0xfd, 0x8a, 0xa8, 0xe4, 0x5f, 0x31, 0xab, 0x96,
+	0x29, 0x01, 0x7d, 0x0e, 0x43, 0x23, 0x0f, 0xae, 0xc2, 0x1c, 0x05, 0x79, 0x04, 0x10, 0x6f, 0x13,
+	0xcc, 0xa4, 0x8f, 0x42, 0x56, 0x14, 0x47, 0x11, 0xda, 0x85, 0xd3, 0x79, 0xba, 0x93, 0xb7, 0xf4,
+	0x07, 0x8c, 0x96, 0x3c, 0x97, 0x3e, 0xcf, 0xa4, 0x88, 0x62, 0x69, 0xa6, 0x13, 0xb0, 0x37, 0x51,
+	0xbe, 0xd1, 0x9d, 0x03, 0xa6, 0xcf, 0x64, 0x02, 0xbd, 0x9b, 0x64, 0x8b, 0x59, 0x94, 0x1a, 0x2d,
+	0x0e, 0x98, 0x3c, 0x80, 0x4e, 0x9e, 0xac, 0x33, 0x14, 0x6e, 0xfb, 0xa2, 0x5d, 0x64, 0x2a, 0xa4,
+	0xe4, 0x8b, 0x79, 0x9a, 0x16, 0x63, 0x5d, 0xbb, 0x94, 0xaf, 0x82, 0x85, 0x7c, 0xe3, 0x8f, 0x3c,
+	0xc9, 0x82, 0xa2, 0x2e, 0x92, 0x7b, 0x81, 0x66, 0x32, 0x85, 0x41, 0x5c, 0x2d, 0x13, 0xee, 0x93,
+	0x55, 0xb5, 0x7b, 0x2d, 0xa6, 0xb6, 0xdb, 0x71, 0x51, 0x5a, 0x7c, 0xce, 0xf4, 0x99, 0xbe, 0x02,
+	0x87, 0x61, 0xb4, 0xba, 0x55, 0x84, 0x77, 0xe0, 0x9a, 0xfd, 0x6a, 0x41, 0x6f, 0xb9, 0x8d, 0xe4,
+	0x0d, 0x17, 0x29, 0x99, 0x41, 0xcf, 0x08, 0x49, 0xc6, 0xfa, 0x51, 0xfc, 0xf5, 0xec, 0x26, 0xc3,
+	0xfa, 0x53, 0xa1, 0x27, 0xe4, 0x19, 0xd8, 0xca, 0x37, 0xe2, 0xe8, 0xcc, 0x91, 0x85, 0x93, 0x51,
+	0x8d, 0xa1, 0x74, 0xa6, 0x68, 0x78, 0x0a, 0x10, 0x66, 0xc2, 0x8c, 0x81, 0x92, 0x50, 0x99, 0xd1,
+	0x40, 0xfe, 0x06, 0x06, 0xc7, 0xf6, 0x10, 0x57, 0x57, 0x34, 0x38, 0xd6, 0xd0, 0xfb, 0x16, 0xce,
+	0x6b, 0x0a, 0x93, 0x87, 0xba, 0xa4, 0x49, 0xf5, 0x86, 0xee, 0x97, 0xd0, 0x3f, 0xe8, 0x49, 0xee,
+	0x57, 0x37, 0xa9, 0xeb, 0xfb, 0x6f, 0xd7, 0x97, 0x8e, 0xfe, 0xd3, 0xbe, 0xf8, 0x13, 0x00, 0x00,
+	0xff, 0xff, 0x35, 0x7d, 0xdf, 0xdc, 0xca, 0x03, 0x00, 0x00,
 }
diff --git a/dfssp/api/gRPCServices.proto b/dfssp/api/gRPCServices.proto
index f02fe401d68bd20311af31eec72471c85fcb5465..fd6888374755d5d7ff8bcb3794b95e13873c3118 100644
--- a/dfssp/api/gRPCServices.proto
+++ b/dfssp/api/gRPCServices.proto
@@ -57,7 +57,7 @@ message Empty {
 // PostContractRequest message contains the contract as SHA-512 hash, its filename,
 // the list of signers as an array of strings, and a comment
 message PostContractRequest {
-	string hash = 1;
+	bytes hash = 1;
 	string filename = 2;
 	repeated string signer = 3;
 	string comment = 4;
diff --git a/dfssp/contract/contract_test.go b/dfssp/contract/contract_test.go
index ba24d33da195d2182b676c388c3f752451527a83..61b57974657ba9cc02f48a05318132315c120f8c 100644
--- a/dfssp/contract/contract_test.go
+++ b/dfssp/contract/contract_test.go
@@ -56,8 +56,8 @@ func TestAddSigner(t *testing.T) {
 
 	id := bson.NewObjectId()
 
-	c.AddSigner(nil, "mail1", "hash1")
-	c.AddSigner(&id, "mail2", "hash2")
+	c.AddSigner(nil, "mail1", []byte{0xaa})
+	c.AddSigner(&id, "mail2", []byte{})
 
 	signers := c.Signers
 
@@ -66,10 +66,10 @@ func TestAddSigner(t *testing.T) {
 	}
 
 	assert.Equal(t, signers[0].Email, "mail1")
-	assert.Equal(t, signers[0].Hash, "hash1")
+	assert.Equal(t, signers[0].Hash, []byte{0xaa})
 	assert.Equal(t, signers[0].UserID.Hex(), "000000000000000000000000")
 	assert.Equal(t, signers[1].Email, "mail2")
-	assert.Equal(t, signers[1].Hash, "hash2")
+	assert.Equal(t, signers[1].Hash, []byte{})
 	assert.Equal(t, signers[1].UserID.Hex(), id.Hex())
 }
 
@@ -85,10 +85,10 @@ func assertContractEqual(t *testing.T, contract, fetched entities.Contract) {
 func TestInsertContract(t *testing.T) {
 	dropDataset()
 	c := entities.NewContract()
-	c.AddSigner(nil, "mail1", "hash1")
-	c.AddSigner(nil, "mail1", "hash1")
+	c.AddSigner(nil, "mail1", []byte{0xaa})
+	c.AddSigner(nil, "mail1", []byte{0xaa})
 	c.File.Name = "file"
-	c.File.Hash = "hashFile"
+	c.File.Hash = []byte{0xff}
 	c.File.Hosted = false
 	c.Comment = "comment"
 	c.Ready = true
@@ -118,17 +118,17 @@ func TestGetWaitingForUser(t *testing.T) {
 
 	dropDataset()
 	c1 := entities.NewContract()
-	c1.AddSigner(nil, "mail1", "")
+	c1.AddSigner(nil, "mail1", []byte{})
 	c1.Ready = false
 
 	c2 := entities.NewContract()
-	c2.AddSigner(nil, "mail1", "")
-	c2.AddSigner(&knownID, "mail2", "hash")
+	c2.AddSigner(nil, "mail1", []byte{})
+	c2.AddSigner(&knownID, "mail2", []byte{0x12})
 	c2.Ready = false
 
 	c3 := entities.NewContract()
-	c3.AddSigner(nil, "mail2", "")
-	c3.AddSigner(&knownID, "mail1", "hash")
+	c3.AddSigner(nil, "mail2", []byte{})
+	c3.AddSigner(&knownID, "mail1", []byte{0xaa})
 	c3.Ready = false
 
 	_, _ = repository.Collection.Insert(c1)
diff --git a/dfssp/contract/create.go b/dfssp/contract/create.go
index 98a03c7c8bbec8fc2a3c128c3864315c52deab81..840efb7ae899a6e4a5eb16ff3be70217301c7bc8 100644
--- a/dfssp/contract/create.go
+++ b/dfssp/contract/create.go
@@ -70,7 +70,7 @@ func (c *Builder) checkInput() *api.ErrorCode {
 		return &api.ErrorCode{Code: api.ErrorCode_INVARG, Message: "Expecting a valid filename"}
 	}
 
-	if len(c.in.Hash) != sha512.Size*2 { // *2 because string format doubles the length
+	if len(c.in.Hash) != sha512.Size {
 		return &api.ErrorCode{Code: api.ErrorCode_INVARG, Message: "Expecting a valid sha512 hash"}
 	}
 
@@ -117,7 +117,7 @@ func (c *Builder) addContract() error {
 		contract.AddSigner(&s.ID, s.Email, s.CertHash)
 	}
 	for _, s := range c.missingSigners {
-		contract.AddSigner(nil, s, "")
+		contract.AddSigner(nil, s, nil)
 	}
 
 	contract.Comment = c.in.Comment
diff --git a/dfssp/contract/create_test.go b/dfssp/contract/create_test.go
index 59a59cfb31efd04e07bcaffc800df8b83163e622..ca3e70b022f1b5ec9fb504c118329d6c2f0b8c3a 100644
--- a/dfssp/contract/create_test.go
+++ b/dfssp/contract/create_test.go
@@ -2,7 +2,6 @@ package contract_test
 
 import (
 	"crypto/sha512"
-	"fmt"
 	"io/ioutil"
 	"path/filepath"
 	"testing"
@@ -18,7 +17,6 @@ import (
 
 var user1, user2, user3 *entities.User
 var defaultHash = sha512.Sum512([]byte{0})
-var defaultHashStr = fmt.Sprintf("%x", defaultHash)
 
 func createDataset() {
 
@@ -29,17 +27,17 @@ func createDataset() {
 	user1.Email = "user1@example.com"
 	user1.Expiration = time.Now().AddDate(1, 0, 0)
 	user1.Certificate = "Certificate1"
-	user1.CertHash = "Hash1"
+	user1.CertHash = []byte{0x01}
 
 	user2.Email = "user2@example.com"
 	user2.Expiration = time.Now().AddDate(1, 0, 0)
 	user2.Certificate = "Certificate2"
-	user2.CertHash = "Hash2"
+	user2.CertHash = []byte{0x02}
 
 	user3.Email = "user3@example.com"
 	user3.Expiration = time.Now().AddDate(0, 0, -1)
 	user3.Certificate = "Certificate3"
-	user3.CertHash = "Hash3"
+	user3.CertHash = []byte{0x03}
 
 	_, _ = manager.Get("users").Insert(user1)
 	_, _ = manager.Get("users").Insert(user2)
@@ -91,7 +89,7 @@ func TestAddContract(t *testing.T) {
 
 	client := clientTest(t)
 	errorCode, err := client.PostContract(context.Background(), &api.PostContractRequest{
-		Hash:     defaultHashStr,
+		Hash:     defaultHash[:],
 		Filename: "ContractFilename",
 		Signer:   []string{user1.Email, user2.Email},
 		Comment:  "ContractComment",
@@ -107,7 +105,7 @@ func TestAddContract(t *testing.T) {
 	}
 
 	assert.Equal(t, 1, len(contracts))
-	assert.Equal(t, defaultHashStr, contracts[0].File.Hash)
+	assert.Equal(t, defaultHash[:], contracts[0].File.Hash)
 	assert.Equal(t, "ContractFilename", contracts[0].File.Name)
 	assert.Equal(t, "ContractComment", contracts[0].Comment)
 	assert.T(t, contracts[0].Ready)
@@ -127,7 +125,7 @@ func TestAddContractMissingUser(t *testing.T) {
 
 	client := clientTest(t)
 	errorCode, err := client.PostContract(context.Background(), &api.PostContractRequest{
-		Hash:     defaultHashStr,
+		Hash:     defaultHash[:],
 		Filename: "ContractFilename",
 		Signer:   []string{user1.Email, user3.Email},
 	})
@@ -142,7 +140,7 @@ func TestAddContractMissingUser(t *testing.T) {
 	}
 
 	assert.Equal(t, 1, len(contracts))
-	assert.Equal(t, defaultHashStr, contracts[0].File.Hash)
+	assert.Equal(t, defaultHash[:], contracts[0].File.Hash)
 	assert.Equal(t, "ContractFilename", contracts[0].File.Name)
 	assert.Equal(t, "", contracts[0].Comment)
 	assert.T(t, !contracts[0].Ready)
@@ -152,7 +150,7 @@ func TestAddContractMissingUser(t *testing.T) {
 	assert.Equal(t, user1.CertHash, contracts[0].Signers[0].Hash)
 	assert.Equal(t, user1.Email, contracts[0].Signers[0].Email)
 	assert.Equal(t, "000000000000000000000000", contracts[0].Signers[1].UserID.Hex())
-	assert.Equal(t, "", contracts[0].Signers[1].Hash)
+	assert.Equal(t, []byte{}, contracts[0].Signers[1].Hash)
 	assert.Equal(t, user3.Email, contracts[0].Signers[1].Email)
 }
 
@@ -162,7 +160,7 @@ func TestAddContractNoUser(t *testing.T) {
 
 	client := clientTest(t)
 	errorCode, err := client.PostContract(context.Background(), &api.PostContractRequest{
-		Hash:     defaultHashStr,
+		Hash:     defaultHash[:],
 		Filename: "ContractFilename",
 		Signer:   []string{},
 	})
@@ -185,7 +183,7 @@ func TestAddContractDuplicatedUser(t *testing.T) {
 
 	client := clientTest(t)
 	errorCode, err := client.PostContract(context.Background(), &api.PostContractRequest{
-		Hash:     defaultHashStr,
+		Hash:     defaultHash[:],
 		Filename: "ContractFilename",
 		Signer:   []string{user1.Email, user1.Email, user2.Email},
 	})
@@ -209,7 +207,7 @@ func TestAddContractNoFilename(t *testing.T) {
 
 	client := clientTest(t)
 	errorCode, err := client.PostContract(context.Background(), &api.PostContractRequest{
-		Hash:   defaultHashStr,
+		Hash:   defaultHash[:],
 		Signer: []string{user1.Email},
 	})
 	assert.Equal(t, nil, err)
@@ -231,7 +229,7 @@ func TestAddContractBadHash(t *testing.T) {
 
 	client := clientTest(t)
 	errorCode, err := client.PostContract(context.Background(), &api.PostContractRequest{
-		Hash:     "aVeryBadHash",
+		Hash:     []byte{0xba, 0xd},
 		Filename: "ContractFilename",
 		Signer:   []string{user1.Email},
 	})
diff --git a/dfssp/contract/file.go b/dfssp/contract/file.go
index d0b42c2310f56c087e91c767dded13b6ac33e97f..3a088a03f19669196ada3483a660556ec7311157 100644
--- a/dfssp/contract/file.go
+++ b/dfssp/contract/file.go
@@ -2,6 +2,7 @@ package contract
 
 import (
 	"encoding/json"
+	"fmt"
 	"time"
 
 	"dfss/dfssp/entities"
@@ -46,7 +47,7 @@ func GetJSON(c *entities.Contract, ttp *TTPJSON) ([]byte, error) {
 		Comment: c.Comment,
 		File: &FileJSON{
 			Name:   c.File.Name,
-			Hash:   c.File.Hash,
+			Hash:   fmt.Sprintf("%x", c.File.Hash),
 			Hosted: c.File.Hosted,
 		},
 		Signers: make([]SignerJSON, len(c.Signers)),
@@ -55,7 +56,7 @@ func GetJSON(c *entities.Contract, ttp *TTPJSON) ([]byte, error) {
 
 	for i, s := range c.Signers {
 		data.Signers[i].Email = s.Email
-		data.Signers[i].Hash = s.Hash
+		data.Signers[i].Hash = fmt.Sprintf("%x", s.Hash)
 	}
 
 	return json.MarshalIndent(data, "", "  ")
diff --git a/dfssp/contract/file_test.go b/dfssp/contract/file_test.go
index ccc05a938f669ac28e0ed08421a996ba2f8ddcab..5a396f6a349652a9db767623da184e95ca871d84 100644
--- a/dfssp/contract/file_test.go
+++ b/dfssp/contract/file_test.go
@@ -20,12 +20,12 @@ func TestGetJSON(t *testing.T) {
 allow multiline and accents: éÉ`,
 		File: &entities.File{
 			Name:   "filename.pdf",
-			Hash:   "hash",
+			Hash:   []byte{0x01, 0x02},
 			Hosted: false,
 		},
 		Signers: []entities.Signer{
-			entities.Signer{Email: "a", Hash: "ha"},
-			entities.Signer{Email: "b", Hash: "hb"},
+			entities.Signer{Email: "a", Hash: []byte{0xaa}},
+			entities.Signer{Email: "b", Hash: []byte{0xbb}},
 		},
 	}
 
@@ -35,17 +35,17 @@ allow multiline and accents: éÉ`,
   "Comment": "A test comment\nallow multiline and accents: éÉ",
   "File": {
     "Name": "filename.pdf",
-    "Hash": "hash",
+    "Hash": "0102",
     "Hosted": false
   },
   "Signers": [
     {
       "Email": "a",
-      "Hash": "ha"
+      "Hash": "aa"
     },
     {
       "Email": "b",
-      "Hash": "hb"
+      "Hash": "bb"
     }
   ],
   "Sequence": null,
diff --git a/dfssp/entities/contract.go b/dfssp/entities/contract.go
index 9ee1793d626e9a0c19ae2a910a6b707c2924677d..30dffabafb74f5cbbfe1fefcf7c3c734ed2e0ea5 100644
--- a/dfssp/entities/contract.go
+++ b/dfssp/entities/contract.go
@@ -10,7 +10,7 @@ import (
 // File : Represents a file structure
 type File struct {
 	Name   string `key:"name" bson:"name"`     // Name of the File
-	Hash   string `key:"hash" bson:"hash"`     // Hash of the File
+	Hash   []byte `key:"hash" bson:"hash"`     // Hash of the File
 	Hosted bool   `key:"hosted" bson:"hosted"` // True if hosted on the platform, else false
 }
 
@@ -18,7 +18,7 @@ type File struct {
 type Signer struct {
 	UserID bson.ObjectId `key:"userId" bson:"userId"`
 	Email  string        `key:"email" bson:"email"`
-	Hash   string        `key:"hash" bson:"hash"`
+	Hash   []byte        `key:"hash" bson:"hash"`
 }
 
 // Contract : Informations about a contract to be signed
@@ -44,7 +44,7 @@ func NewContract() *Contract {
 }
 
 // AddSigner : Add a signer to the contract
-func (c *Contract) AddSigner(id *bson.ObjectId, email, hash string) {
+func (c *Contract) AddSigner(id *bson.ObjectId, email string, hash []byte) {
 	signer := &Signer{}
 	signer.Email = email
 
@@ -78,7 +78,7 @@ func (r *ContractRepository) GetWaitingForUser(email string) ([]Contract, error)
 		"signers": bson.M{
 			"$elemMatch": bson.M{
 				"email": email,
-				"hash":  "",
+				"hash":  []byte{},
 			}},
 	}, &res)
 	return res, err
diff --git a/dfssp/entities/user.go b/dfssp/entities/user.go
index 2c0a90ebdbc62710e43e51ccd4010b3870ba3045..9d077e6256880367ff30d77a5b3efdf7b3e71354 100644
--- a/dfssp/entities/user.go
+++ b/dfssp/entities/user.go
@@ -28,7 +28,7 @@ type User struct {
 	RegToken     string         `key:"regToken" bson:"regToken"`         // Token used for registering a User
 	Csr          string         `key:"csr" bson:"csr"`                   // Certificate request at PEM format
 	Certificate  string         `key:"certificate" bson:"certificate"`   // Certificate of the User
-	CertHash     string         `key:"certHash" bson:"certHash"`         // Hash of the certificate
+	CertHash     []byte         `key:"certHash" bson:"certHash"`         // Hash of the certificate
 	ConnInfo     ConnectionInfo `key:"connInfo" bson:"connInfo"`         // Information about the connection
 }
 
@@ -54,7 +54,7 @@ func NewUserRepository(collection *mgdb.MongoCollection) *UserRepository {
 }
 
 // FetchByMailAndHash : Fetches a User from its email and certificate hash
-func (repository *UserRepository) FetchByMailAndHash(email, hash string) (*User, error) {
+func (repository *UserRepository) FetchByMailAndHash(email string, hash []byte) (*User, error) {
 	var users []User
 	err := repository.Collection.FindAll(bson.M{"email": email, "certHash": hash}, &users)
 	if err != nil || len(users) == 0 {
diff --git a/dfssp/templates/contract.go b/dfssp/templates/contract.go
index d968b9ef8630eae0fa2e375fdf72ae029e34fc21..0b63c5f4698dff9b9c3990ef160d5172fb917856 100644
--- a/dfssp/templates/contract.go
+++ b/dfssp/templates/contract.go
@@ -13,6 +13,6 @@ const contractDetails = `Signers :
 {{range .Signers}}  - {{.Email}}
 {{end}}
 Contract name : {{.File.Name}}
-SHA-512 hash  : {{.File.Hash}}
+SHA-512 hash  : {{printf "%x" .File.Hash}}
 Comment       : {{.Comment}}
 `
diff --git a/dfssp/templates/manager_test.go b/dfssp/templates/manager_test.go
index e8d289ef80e4d7f6e043649a0f6416ea0b0fb71b..44fbf32e1326fe021e1b149ba9712d4b200bb53c 100644
--- a/dfssp/templates/manager_test.go
+++ b/dfssp/templates/manager_test.go
@@ -14,11 +14,11 @@ func TestInit(t *testing.T) {
 func TestGet(t *testing.T) {
 
 	contract := entities.NewContract()
-	contract.File.Hash = "hash"
+	contract.File.Hash = []byte{0x01, 0x02, 0x11, 0xaa}
 	contract.File.Name = "name.pdf"
 	contract.Comment = "comment"
-	contract.AddSigner(nil, "mail@example.com", "")
-	contract.AddSigner(nil, "mail2@example.com", "")
+	contract.AddSigner(nil, "mail@example.com", nil)
+	contract.AddSigner(nil, "mail2@example.com", nil)
 
 	s, err := Get("contract", contract)
 
@@ -32,7 +32,7 @@ Signers :
   - mail2@example.com
 
 Contract name : name.pdf
-SHA-512 hash  : hash
+SHA-512 hash  : 010211aa
 Comment       : comment
 
 Yours faithfully,
diff --git a/dfssp/user/create.go b/dfssp/user/create.go
index c737c9c1dd1ebf3baa0fd0fb5add86f154d34279..3af4c6ecebf31f8dae8651b173aece2e366f36bf 100644
--- a/dfssp/user/create.go
+++ b/dfssp/user/create.go
@@ -199,7 +199,7 @@ func Auth(pid *authority.PlatformID, manager *mgdb.MongoManager, certDuration in
 	}
 
 	// If the user already has a certificate and certificate hash in the database, does nothing
-	if user.Certificate != "" || user.CertHash != "" {
+	if user.Certificate != "" || len(user.CertHash) != 0 {
 		return nil, errors.New("User is already registered")
 	}
 
@@ -221,7 +221,7 @@ func Auth(pid *authority.PlatformID, manager *mgdb.MongoManager, certDuration in
 	}
 
 	user.Certificate = string(cert)
-	user.CertHash = fmt.Sprintf("%x", certHash)
+	user.CertHash = certHash
 	user.Expiration = time.Now().AddDate(0, 0, certDuration)
 
 	// Updating the database
diff --git a/dfssp/user/user_test.go b/dfssp/user/user_test.go
index df1b8f212f2f4979f72bb0e9fb8353353cbbac18..056ac5ecc2d4a6905be5c9ff2175fd49c0c525ac 100644
--- a/dfssp/user/user_test.go
+++ b/dfssp/user/user_test.go
@@ -75,11 +75,8 @@ func TestMain(m *testing.M) {
 	go func() { _ = net.Listen(InvalidServ, srv2) }()
 
 	// Run
+	err = collection.Drop()
 	code := m.Run()
-
-	// Teardown
-	// The collection is created automatically on
-	// first connection, that's why we do not recreate it manually
 	err = collection.Drop()
 
 	if err != nil {
@@ -91,7 +88,7 @@ func TestMain(m *testing.M) {
 }
 
 func TestMongoFetchInexistantUser(t *testing.T) {
-	user, erro := repository.FetchByMailAndHash("dummyMail", "dummyHash")
+	user, erro := repository.FetchByMailAndHash("dummyMail", []byte{0x01})
 	if user != nil || erro != nil {
 		t.Fatal("User should not have been found and error should  be nil")
 	}
@@ -100,7 +97,7 @@ func TestMongoFetchInexistantUser(t *testing.T) {
 func TestMongoInsertUser(t *testing.T) {
 	user := entities.NewUser()
 	user.Email = "dfss1@mpcs.tk"
-	user.CertHash = "dummy_hash"
+	user.CertHash = []byte{0x01, 0x02}
 	user.ConnInfo.IP = "127.0.0.1"
 	user.ConnInfo.Port = 1111
 	user.Csr = "csr1"
@@ -117,7 +114,7 @@ func equalUsers(t *testing.T, user1, user2 *entities.User) {
 		t.Fatal("ID doesn't match : received ", user1.ID, " and ", user2.ID)
 	}
 
-	if user1.CertHash != user2.CertHash {
+	if string(user1.CertHash) != string(user2.CertHash) {
 		t.Fatal("CertHash doesn't match : received ", user1.CertHash, " and ", user2.CertHash)
 	}
 
@@ -153,7 +150,7 @@ func equalUsers(t *testing.T, user1, user2 *entities.User) {
 func TestMongoFetchUser(t *testing.T) {
 	user := entities.NewUser()
 	user.Email = "dfss2@mpcs.tk"
-	user.CertHash = "dummy_hash"
+	user.CertHash = nil
 	user.ConnInfo.IP = "127.0.0.2"
 	user.ConnInfo.Port = 2222
 	user.Csr = "csr2"
@@ -240,7 +237,7 @@ func TestWrongAuthRequestContext(t *testing.T) {
 	}
 
 	assert.Equal(t, res.Certificate, "")
-	assert.Equal(t, res.CertHash, "")
+	assert.Equal(t, res.CertHash, []byte{})
 
 	// Invalid certificate request (none here)
 	request.Token = token
@@ -255,7 +252,7 @@ func TestWrongAuthRequestContext(t *testing.T) {
 		t.Fatal(err)
 	}
 	assert.Equal(t, res.Certificate, "")
-	assert.Equal(t, res.CertHash, "")
+	assert.Equal(t, res.CertHash, []byte{})
 }
 
 func ExampleAuth() {
@@ -305,7 +302,7 @@ func ExampleAuth() {
 		fmt.Println(err)
 	}
 
-	if res.Certificate == "" || res.CertHash == "" {
+	if res.Certificate == "" || res.CertHash == nil {
 		fmt.Println("The database should have been updated")
 	}
 
@@ -347,7 +344,7 @@ func TestAuthTwice(t *testing.T) {
 	user.RegToken = token
 	user.Csr = string(csr)
 	user.Certificate = "foo"
-	user.CertHash = "foo"
+	user.CertHash = []byte{0xaa}
 
 	_, err = repository.Collection.Insert(*user)
 	if err != nil {
diff --git a/tests/new_test.go b/tests/new_test.go
index cdc417112a60512c443192cdea8a61db9fb554d4..41a9e79a6a9177a37197ed2b67909bde861988bc 100644
--- a/tests/new_test.go
+++ b/tests/new_test.go
@@ -1,6 +1,7 @@
 package tests
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -63,7 +64,7 @@ func TestNewContract(t *testing.T) {
 	contract := getContract("contract.txt", 0)
 	assert.Equal(t, false, contract.Ready)
 	assert.Equal(t, "A very nice comment", contract.Comment)
-	assert.Equal(t, "6a95f6bcd6282186a7b1175fbaab4809ca5f665f7c4d55675de2399c83e67252069d741a88c766b1a79206d6dfbd5552cd7f9bc69b43bee161d1337228b4a4a8", contract.File.Hash)
+	assert.Equal(t, "6a95f6bcd6282186a7b1175fbaab4809ca5f665f7c4d55675de2399c83e67252069d741a88c766b1a79206d6dfbd5552cd7f9bc69b43bee161d1337228b4a4a8", fmt.Sprintf("%x", contract.File.Hash))
 	assert.Equal(t, 2, len(contract.Signers))
 	assert.Equal(t, "client1@example.com", contract.Signers[0].Email)
 	assert.Equal(t, "client2@example.com", contract.Signers[1].Email)