I need to sign the message using ES256 algorithm. After doing necessary initializations, I called API - mbedtls_ecdsa_write_signature() API and it gave me signature in ASN1 encoded form and there was no error generated by this API. 

After getting the signature, I need the r & s values to create JWT Token. So, I wrote my custom function to parse the signature buffer and get the R & S values of it. 

It was working fine. Sometimes, I am getting an invalid signature as shown below signature DER buffer -

30 43 02 1f 31 92 8d 22 10 41 86 25 68 7f 42 81 26 0f 37 bc 7f 38 b7 d5 1a 6b 69 31 07 34 11 a6 04 e5 90 02 20 23 26 f8 b9 80 cf 2c 25 c8 04 b4 ac 43 51 6a 04 a6 af 8f 94 36 f8 cf 35 c2 94 cc df de db 92 b2

The reason for invalid is -

1st byte represents ASN1 sequence, followed by length and 3rd byter indicates it is an integer. 

Ideally, 4th byte indicates length of r-value, it should have been 32 or 33 bytes ( in case of padding with 00 ). You can see in the above buffer it is 0x1F ( 31 bytes ). It is really weird how it is possible to get the signature length of 31 bytes.

It is blocking me for generation of JWT token, where in RFC 7518 - https://tools.ietf.org/html/rfc7518#page-9 , it says R & S must be 32 bytes long. And, the generation is failing. 

It is of high priority for me. If anyone can provide your suggestions on this issue, it would be really great. Thanks in advance

Thanks,

Roshini