We all know that x509 certificates carry a signature that represents the certificate digest encrypted by the private key of the issuer.
I believe that the digest is encrypted only providing one block, so it will represent an unique congruence in the multiplicative group space. It doesn't matter if two digest are very similar. Hash functions are used to reduce the message length, so data has not to be separated in chunks since length is less than the modulus length.
But I started to wondering what would happen if the CA enciphers the digest chunk by chunk.
For example we could obtain a similar digest of a legit certificate that differs in the last $n$ bits. So we will have almost the same congruences (ciphertext) comparing to the original signature. This ends in brute forcing the different chunks. I know that this isn't the real case, and also, that nobody ciphers a message that is bigger in size than the modulus.
As a consecuence, are hash functions used to reduce the message length so its less in size than the modulus? Can be the data partitioned in chunks and then ciphered by the private key when dealing with signatures?