ECDSA read_signature returns "invalid signature" status - mbed-tls

4 Mar 2023


      Hello,
I have 3rd party custom ECC library, that can do ECDSA verification and uses secp256r1 compressed public key (33bytes) to do so - all works fine.
Now I want to migrate to mbedTLS, to also benefit of other crypto schemes, hence use of mbedtls ECDSA was a natural way to go.
Here I need (as I understand) PEM parser or optionally public key in uncompressed format (0x04 | X | Y).
Problem is that loading of the key seems to work (func returns 0), but verification fails with -20450, indicating (if I well understood) invalid signature.
Test data.
PRIVATE KEY:
-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgKNqyWso/lMuTlTE6
ll47Jboqq/Iz7OYDrr7TuXN+s2ChRANCAARNgfaUcxLoWWG01ekJFiqB8ujMgnHz
P320ZgiZErH6zKjlB9EovIHrchj0240+EIpFios+2uM609FgRvu3+NrT
-----END PRIVATE KEY-----
PUBLIC KEY:
-----BEGIN PUBLIC KEY-----
MDkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDIgADTYH2lHMS6FlhtNXpCRYqgfLozIJx
8z99tGYImRKx+sw=
-----END PUBLIC KEY-----
PUBLIC KEY UNCOMPRESSED SEC1: 
0x4,0x4d,0x81,0xf6,0x94,0x73,0x12,0xe8,0x59,0x61,0xb4,0xd5,0xe9,0x9,0x16,0x2a,0x81,0xf2,0xe8,0xcc,0x82,0x71,0xf3,0x3f,0x7d,0xb4,0x66,0x8,0x99,0x12,0xb1,0xfa,0xcc,0xa8,0xe5,0x7,0xd1,0x28,0xbc,0x81,0xeb,0x72,0x18,0xf4,0xdb,0x8d,0x3e,0x10,0x8a,0x45,0x8a,0x8b,0x3e,0xda,0xe3,0x3a,0xd3,0xd1,0x60,0x46,0xfb,0xb7,0xf8,0xda,0xd3
PUBLIC KEY COMPRESSED SEC1:
0x3,0x4d,0x81,0xf6,0x94,0x73,0x12,0xe8,0x59,0x61,0xb4,0xd5,0xe9,0x9,0x16,0x2a,0x81,0xf2,0xe8,0xcc,0x82,0x71,0xf3,0x3f,0x7d,0xb4,0x66,0x8,0x99,0x12,0xb1,0xfa,0xcc
INPUT STRING in TEXT format:
"This is my input data" (remove quotes)
INPUT STRING in HEX format:
0x54,0x68,0x69,0x73,0x20,0x69,0x73,0x20,0x6d,0x79,0x20,0x69,0x6e,0x70,0x75,0x74,0x20,0x64,0x61,0x74,0x61
SHA256 of INPUT STRING:
0xa7,0x3f,0x26,0xf4,0xa1,0xe4,0x61,0x61,0x0,0x1a,0x29,0xdf,0xd2,0xaf,0x7d,0xa,0x25,0x91,0xbb,0xcc,0x1f,0xbc,0xfb,0xdb,0x43,0xdb,0x57,0xf9,0x8d,0x94,0xeb,0x81
(x-checked here: https://emn178.github.io/online-tools/sha256.html)
SIGNATURE of HASH signed with PRIVATE KEY:
0x80,0xe6,0xf5,0x97,0x6a,0x66,0xa2,0xe2,0x9a,0xd7,0x7f,0x9a,0x9b,0x3e,0x2b,0xde,0x1f,0x7c,0x3,0xb3,0x1,0xb8,0x6f,0xd8,0xf6,0xf,0x27,0x38,0x63,0x3,0x54,0x74,0x76,0x6d,0x1b,0x97,0xf0,0xbc,0xc5,0xd2,0x4b,0xae,0xf0,0x34,0xab,0x86,0xbd,0x55,0x0,0x8a,0x4c,0x9f,0x4e,0xa5,0x53,0x89,0xe8,0x0,0xb9,0x83,0x24,0x87,0x98,0x1
My custom library code looks like - this one works as expected:
if (ecdsa_verify(public_key_compressed_33_bytes_array, hash_of_input_string, signature_signed_with_private_key)) {
    printf("Custom ECDSA lib verification is OK\r\n");
}
My mbedTLS code looks like:
```
    /* mbedTLS */
    printf("mbedTLS way start\r\n");
    mbedtls_ecdsa_init(&ctx);
    mbedtls_ecp_group_load(&ctx.private_grp, MBEDTLS_ECP_DP_SECP256R1);
    res = mbedtls_ecp_point_read_binary(&ctx.private_grp, &ctx.private_Q, ecc_public_key_uncompressed_bin,
                                        sizeof(ecc_public_key_uncompressed_bin));
    if (res != 0) {
        printf("ECP point read binary failed: %d\r\n", res);
    }
res = mbedtls_ecdsa_read_signature(&ctx, data_raw_hash_digest, sizeof(data_raw_hash_digest), signature,
                                       sizeof(signature));
    if (res == 0) {
        printf("mbedTLS Verification is OK...\r\n");
    } else {
        printf("mbedTLS Verification failed...: %d\r\n", res);
    }
    printf("mbedTLS way end\r\n");
```
and it always fails with error code -20450. while loading keys function goes through well.
Am I wrongly loading the keys?