Here is a list of all files with brief descriptions:

include
osmocom
crypt
auth.h
gprs_cipher.h
kdf.h
utran_cipher.h
gsm
protocol
gsm_03_40.h	GSM TS 03.40
gsm_03_41.h	GSM TS 03.41 definitions also TS 23.041
gsm_04_08.h	GSM TS 04.08 definitions
gsm_04_08_gprs.h
gsm_04_11.h
gsm_04_12.h	GSM TS 04.12 definitions for Short Message Service Cell Broadcast
gsm_04_14.h
gsm_04_80.h	GSM TS 04.80 definitions (Supplementary Services Specification, Formats and Coding)
gsm_08_08.h
gsm_08_58.h	GSM Radio Signalling Link messages on the A-bis interface
gsm_09_02.h	GSM TS 09.02 definitions (MAP)
gsm_12_21.h
gsm_23_003.h
gsm_23_032.h
gsm_23_041.h
gsm_25_415.h
gsm_29_118.h
gsm_44_004.h
gsm_44_060.h	General Packet Radio Service (GPRS)
gsm_44_068.h
gsm_44_318.h	Definitions according to 3GPP TS 44.318 6.8.0 Release 6
gsm_48_049.h
gsm_48_071.h
gsm_48_103.h
gsm_49_031.h
ipaccess.h
smpp34_osmocom.h	Osmocom SMPP extensions
a5.h
abis_nm.h
apn.h
bitvec_gsm.h
bsslap.h	Message encoding and decoding for 3GPP TS 48.071 BSS LCS Assistance Protocol (BSSLAP)
bssmap_le.h	Message encoding and decoding for 3GPP TS 49.031 BSSMAP-LE
bts_features.h
cbsp.h
comp128.h	COMP128 header
comp128v23.h	COMP128v23 header
gad.h	Message encoding and decoding for 3GPP TS 23.032 GAD: Universal Geographical Area Description
gan.h
gea.h	GEA3/GEA4 header
gsm0341.h
gsm0411_smc.h
gsm0411_smr.h
gsm0411_utils.h
gsm0480.h
gsm0502.h
gsm0503.h
gsm0808.h
gsm0808_lcs.h	Declarations that depend on both gsm0808.h and bssmap_le.h: LCS related message coding
gsm0808_utils.h
gsm23003.h
gsm23236.h	API to handle Network Resource Indicator (NRI) values and ranges for MSC pooling, as in 3GPP TS 23.236
gsm29118.h
gsm29205.h
gsm44021.h
gsm48.h
gsm48_arfcn_range_encode.h
gsm48_ie.h
gsm48_rest_octets.h
gsm_utils.h	GSM utility functions, e.g
gsup.h	Osmocom Generic Subscriber Update Protocol message encoder/decoder
gsup_sms.h	SMS (Short Message Service) extensions for Osmocom GSUP
i460_mux.h
ipa.h
iuup.h
kasumi.h	KASUMI header
l1sap.h
lapd_core.h
lapdm.h
meas_rep.h
mncc.h
oap.h
oap_client.h
prim.h
rlp.h
rsl.h
rtp_extensions.h
rxlev_stat.h
sysinfo.h
tlv.h
src
gsm
kdf
common.h
crypto.h
sha1-internal.c
sha1.c
sha1.h
sha1_i.h
sha256-internal.c
sha256.c
sha256.h
sha256_i.h
milenage
aes-encblock.c	AES encrypt_block
aes-internal-enc.c	AES (Rijndael) cipher - encrypt
aes-internal.c	AES (Rijndael) cipher
aes.h	AES functions
aes_i.h	AES (Rijndael) cipher
aes_wrap.h	AES-based functions
common.h
crypto.h
includes.h
milenage.c	3GPP AKA - Milenage algorithm (3GPP TS 35.205, .206, .207, .208)
milenage.h	UMTS AKA - Milenage algorithm (3GPP TS 35.205, .206, .207, .208)
tuak
KeccakP-1600-3gpp.c
KeccakP-1600-3gpp.h
tuak.c
tuak.h
a5.c
abis_nm.c
apn.c	(C) 2014 by Harald Welte lafor.nosp@m.ge@g.nosp@m.numon.nosp@m.ks.o.nosp@m.rg (C) 2015,2017 by sysmocom - s.f.m.c
auth_comp128v1.c	GSM/GPRS/3G authentication core infrastructure
auth_comp128v23.c	Registers COMP128 version 2 and 3 A3/A8 algorithms for the GSM/GPRS/3G authentication core infrastructure
auth_core.c
auth_milenage.c	GSM/GPRS/3G authentication core infrastructure
auth_tuak.c	GSM/GPRS/3G authentication core infrastructure
auth_xor.c	GSM/GPRS/3G authentication core infrastructure
auth_xor_2g.c
bsslap.c	Message encoding and decoding for 3GPP TS 48.071 BSSLAP protocol
bssmap_le.c	Message encoding and decoding for 3GPP TS 49.031 BSSMAP-LE
bts_features.c	Osmo-bts features
cbsp.c
comp128.c	COMP128 v1; common/old GSM Authentication Algorithm (A3/A8)
comp128v23.c	COMP128 version 2 and 3 implementation, common algorithm used for GSM Authentication (A3/A8)
gad.c	Message encoding and decoding for 3GPP TS 23.032 GAD: Universal Geographical Area Description
gan.c	Generic Access Network (GAN) / UMA according to TS 44.318
gea.c
gprs_cipher_core.c	GPRS LLC cipher core infrastructure
gprs_gea.c	GEA 3 & 4 plugin
gprs_rlc.c	Helper functions for (E)GPRS RLC according to 3GPP TS 44.060
gsm0341.c
gsm0411_smc.c	Point-to-Point (PP) Short Message Service (SMS) as per TS 04.11
gsm0411_smr.c
gsm0411_utils.c
gsm0414.c
gsm0480.c	Format functions for GSM 04.80
gsm0502.c	Paging helper code
gsm0503_conv.c
gsm0808.c	Helper functions regarding the TS 08.08 / 48.008 A interface, primarily message generation/encoding
gsm0808_utils.c
gsm23003.c	Utility function implementations related to 3GPP TS 23.003
gsm23236.c	Utility function implementations related to 3GPP TS 23.236
gsm29118.c
gsm29205.c	Functions related to 3GPP TS 29.205, primarily message generation/encoding
gsm44021.c
gsm44068.c
gsm48.c	GSM Mobile Radio Interface Layer 3 messages 3GPP TS 04.08 version 7.21.0 Release 1998 / ETSI TS 100 940 V7.21.0
gsm48049.c
gsm48_arfcn_range_encode.c
gsm48_ie.c	GSM Mobile Radio Interface Layer 3 messages
gsm48_rest_octets.c
gsm_04_08_gprs.c
gsm_utils.c
gsup.c	Osmocom Generic Subscriber Update Protocol
gsup_sms.c	SMS (Short Message Service) extensions for Osmocom GSUP
ipa.c	OpenBSC Abis input driver for ip.access
iuup.c
kasumi.c	Kasumi cipher and KGcore functions
kdf.c
lapdm.c	GSM LAPDm (TS 04.06) implementation
mncc.c
oap.c	Osmocom Authentication Protocol
oap_client.c
rlp.c
rsl.c
rxlev_stat.c	Rx Level statistics
sysinfo.c	GSM 04.08 System Information (SI) encoding and decoding
tlv_parser.c