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I am using the bicoinjs-message npm module for signature verification.

I learned that there are few signatures which are of length 138 in hexadecimal.
When I tried to verify such a signature it is throwing an error:

Error: Invalid signature length

How to verify the compressed signature?
I think there is a lib in ruby, bitcoin-ruby 0.0.20, which verifies it, but I don't know ruby.

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If you have the bitcoin core software, you should be able to use the CLI to verify signatures using the verifymessage command. Some wallet software also has this functionality.

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There are no compressed signatures. Bitcoin uses two different formats (encodings) for signatures:

  • transaction signatures use the ASN.1 DER encoding, which is variable length depending on the curve and was common when Satoshi published, and adds a 'sighash' byte. For secp256k1 in general, DER(ECDSA-Sig) is usually 72-70 bytes and sometimes but rarely less, but modern bitcoin requires 'low-S' signatures which reduces the maximum to 71. A 69-byte (138-hexit) signature excluding sighash would occur about 3/512 (about one-half a percent) of the time; that length including sighash would be much rarer.

  • message signatures use the P1363 encoding, also called 'plain', 'PKCS11', or 'CVC', which for secp256k1 is always exactly 64 bytes, and adds a recovery byte for a total of 65 (always). It also addes a prefix to the signed data, which changes the value(s) but not the format of the signature.

See e.g.:
Why the signature is always 65 (1+32+32) bytes long?
ECDSA r, s encoding as a signature
Difference between Sign Message and Sign Transaction

Though I haven't looked at it, I expect a library named bitcoinjs-message likely handles signatures for messages not transactions, and any signature that is 69 bytes cannot be a message signature.

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  • Actually, there is a bitcoin-ruby library in ruby. This lib generates the 138 length message signature.
    – Shubham Chadokar
    Aug 12, 2020 at 11:08
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    A bitcoin message signature cannot ever be '138 length'. Not ever in the entire lifetime of the universe. Something named 'bitcoin-ruby' probably handles transactions, which are the original, core, and main function and purpose of bitcoin -- messages are rare and nearly useless.
    – dave_thompson_085
    Aug 14, 2020 at 7:59

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