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6.2 Used S-expressions

Libgcrypt’s API for asymmetric cryptography is based on data structures called S-expressions (see http://people.csail.mit.edu/rivest/sexp.html) and does not work with contexts/handles as most of the other building blocks of Libgcrypt do.

The following information are stored in S-expressions:

• keys
• plain text data
• encrypted data
• signatures

To describe how Libgcrypt expect keys, we use examples. Note that words in uppercase indicate parameters, whereas lowercase words are literals.

Note that all MPI (multi-precision-integers) values are expected to be in GCRYMPI_FMT_USG format. An easy way to create S-expressions is by using gcry_sexp_build which allows to pass a string with printf-like escapes to insert MPI values.

• RSA key parameters
• DSA key parameters
• ECC key parameters