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From the OpenSSL Security Advisory - 0-byte record padding oracle (CVE-2019-1559)

In order for this to be exploitable "non-stitched" ciphersuites must be in use.

What is a 'non-stitched' ciphersuite ?

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The very next sentence gives a short explanation:

Stitched ciphersuites are optimised implementations of certain commonly used ciphersuites.

Googling, the term is better explained in the paper Improving OpenSSL* performance, which cites Fast Cryptographic computation on IA processors via Function Stitching as the source of the term, where you can find:

We present fast and efficient methods of computing such pairs of functions on IA processors using a method called “function stitching”. Instead of computing pairs of functions sequentially as is done today in applications/libraries, we replace the function calls by a single call to a composite function that implements both algorithms. The execution time of this composite function can be made significantly shorter than the sums of the execution times for the individual functions and, in many cases, close to the execution time of the slower function.

As explained in the first paper, the two functions being "stitched" together here are an encryption function and an authentication function, doing both at once instead of one and then the other.

A "non-stitched" implementation would be one that doesn't utilize this sort of performance optimization.

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