The digestMethod object lists an array of acceptable digest algorithms to use while signing. The valid values for PDF 1.7 are SHA1, SHA256, SHA384, SHA512 and RIPEMD160.This object applies only if the digital credential that is signing contains RSA public/private keys. If it contains DSA public/private keys, then the digest algorithm is always SHA1 and this object is ignored. The default value, if not specified, is implementation-specific.