The current design of tls_sign_cb provides a pointer to a buffer where the signature needs to be copied, however it fails to provide a length which could result in buffer overwrites. Furthermore, tls_signer_sign() is designed such that it allocates and returns ownership to the caller. Revise tls_sign_cb so that the called function is expected to allocate a buffer, returning ownership of the buffer (along with its length) to the caller of the callback. This makes it far easier (and safer) to implement a tls_sign_cb callback, plus tls_signer_sign can be directly plugged in (with an appropriate cast). While here, rename and reorder some arguments - while we will normally sign a digest, there is no requirement for this to be the case hence use 'input' and 'input_len'. Move padding (an input) before the outputs and add some additional bounds/return value checks. This is technically an API/ABI break that would need a libtls major bump, however since nothing is using the signer interface (outside of regress), we'll ride the original minor bump. With input from tb@ ok inoguchi@ tb@