target = "https://www.rfc-editor.org/rfc/rfc9001#section-5.4.2"

# 5.4.2.  Header Protection Sample
#
# The header protection algorithm uses both the header protection key
# and a sample of the ciphertext from the packet Payload field.
# 
# The same number of bytes are always sampled, but an allowance needs
# to be made for the removal of protection by a receiving endpoint,
# which will not know the length of the Packet Number field.  The
# sample of ciphertext is taken starting from an offset of 4 bytes
# after the start of the Packet Number field.  That is, in sampling
# packet ciphertext for header protection, the Packet Number field is
# assumed to be 4 bytes long (its maximum possible encoded length).
# 
# An endpoint MUST discard packets that are not long enough to contain
# a complete sample.
# 
# To ensure that sufficient data is available for sampling, packets are
# padded so that the combined lengths of the encoded packet number and
# protected payload is at least 4 bytes longer than the sample required
# for header protection.  The cipher suites defined in [TLS13] -- other
# than TLS_AES_128_CCM_8_SHA256, for which a header protection scheme
# is not defined in this document -- have 16-byte expansions and
# 16-byte header protection samples.  This results in needing at least
# 3 bytes of frames in the unprotected payload if the packet number is
# encoded on a single byte, or 2 bytes of frames for a 2-byte packet
# number encoding.
# 
# The sampled ciphertext can be determined by the following pseudocode:
# 
# # pn_offset is the start of the Packet Number field.
# sample_offset = pn_offset + 4
# 
# sample = packet[sample_offset..sample_offset+sample_length]
# 
# Where the packet number offset of a short header packet can be
# calculated as:
# 
# pn_offset = 1 + len(connection_id)
# 
# And the packet number offset of a long header packet can be
# calculated as:
# 
# pn_offset = 7 + len(destination_connection_id) +
#                 len(source_connection_id) +
#                 len(payload_length)
# if packet_type == Initial:
#     pn_offset += len(token_length) +
#                  len(token)
# 
# For example, for a packet with a short header, an 8-byte connection
# ID, and protected with AEAD_AES_128_GCM, the sample takes bytes 13 to
# 28 inclusive (using zero-based indexing).
# 
# Multiple QUIC packets might be included in the same UDP datagram.
# Each packet is handled separately.

[[spec]]
level = "MUST"
quote = '''
An endpoint MUST discard packets that are not long enough to contain
a complete sample.
'''