I am testing my OpenVPN server [Debian 10, 1 CPU core, 2 GB RAM] and I want to use stronger encryption, but throughput is slow with a stronger encryption cipher [SHA512, AES-256] and if I change the cipher [SHA256, AES-128], throughput is faster.
First, you should split ciphers with usge:
Parameters you have chosen are pretty strong. If you can trade security over speed, using AES-128 with SHA256 is also acceptable.
If you are ok with upgrading sever spec I would recommend AES-256-GCM.
...I want to use stronger encryption, but throughput is slow with a stronger encryption cipher [SHA512, AES-256] and if I change the cipher [SHA256, AES-128], throughput is faster.
Anything over AES128 is pointless since it will remain uncrackable until at least 2030 (similar math applies: all AES192 or AES256 will do is slow throughput without an AES-NI CPU, with no additional gain in security); instead, setting a low key re-negotiation value would be recommended to further harden security:
reneg-bytes <#>: Renegotiate data channel key after # bytes sent/received (Default: disabled)reneg-pkts <#>: Renegotiate data channel key after # packets sent/received (Default: disabled)reneg-sec <#>: Renegotiate data channel key after # seconds (Default: 3600) Encryption/decryption is CPU intensive, so the stronger the cipher, the slower the throughput, and with a single-core CPU, throughput efficiency becomes vital:
# Server config (see: openssl ciphers -v ; openssl ciphers -tls1_2)
HMAC:
auth 'SHA512'
SSL:
cipher 'AES-128-CBC'
TLS (!: specifies disabled ciphers)
tls-version-min '1.2'
tls-crypt '/path/to//openvpn/tls-crypt.psk'
tls-cipher 'TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256:TLS-ECDHE-RSA-WITH-AES-128-CBC-SHA256:TLS-DHE-RSA-WITH-AES-128-CBC-SHA256:TLS-ECDH-RSA-WITH-AES-128-GCM-SHA256:!aNULL:!eNULL:!LOW:!3DES:!MD5:!SHA:!EXP:!PSK:!SRP:!DSS:!RC4:!kRSA'
keyUsage determines what encryption algorithms can be used Cipher speed examples:
openssl list -digest-commands | openssl list -cipher-commands
openssl speed -elapsed aes-128-cbc aes-192-cbc aes-256-cbc
# The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes 16384 bytes
aes-128 cbc 50944.22k 55614.08k 57525.85k 58109.61k 58589.18k 59053.40k
aes-192 cbc 45287.47k 49316.10k 50979.41k 51186.35k 51503.10k 51478.53k
aes-256 cbc 39510.03k 42003.80k 43466.67k 43816.28k 43881.81k 43876.35k
AES-NI CPUs:
openssl speed -elapsed -evp <cipher>
aes-128 cbc 43612.99k 54396.71k 58581.25k 59807.06k 60222.12k 60347.73k
aes-192 cbc 45327.42k 48932.03k 50659.75k 51033.09k 51112.62k 51030.70k
aes-256 cbc 39232.39k 41801.90k 43211.86k 43575.30k 43627.86k 43537.75k
aes-128-gcm 32829.30k 39914.79k 44297.47k 48705.19k 49851.05k 49728.98k
aes-192-gcm 29934.27k 35889.30k 39292.33k 43299.84k 44348.76k 44307.80k
aes-256-gcm 27591.53k 31829.29k 35027.46k 38921.56k 40135.34k 40042.50k
openssl speed -elapsed rsa
type sign verify sign/s verify/s
rsa 512 bits 0.000419s 0.000034s 2387.4 29407.8
rsa 1024 bits 0.002078s 0.000091s 481.1 10943.3
rsa 2048 bits 0.012328s 0.000304s 81.1 3294.0
rsa 3072 bits 0.035587s 0.000638s 28.1 1566.2
rsa 4096 bits 0.077231s 0.001100s 12.9 908.8
openssl speed -elapsed ecdsa
type sign verify sign/s verify/s
192 bits ecdsa (nistp192) 0.0024s 0.0019s 419.6 519.6
224 bits ecdsa (nistp224) 0.0033s 0.0026s 298.8 379.2
256 bits ecdsa (nistp256) 0.0004s 0.0011s 2675.1 923.3
384 bits ecdsa (nistp384) 0.0125s 0.0093s 80.3 108.1
521 bits ecdsa (nistp521) 0.0307s 0.0223s 32.6 44.9
openssl speed -elapsed ecdh
type op op/s
192 bits ecdh (nistp192) 0.0023s 443.9
224 bits ecdh (nistp224) 0.0031s 318.3
256 bits ecdh (nistp256) 0.0008s 1328.1
384 bits ecdh (nistp384) 0.0118s 84.8
521 bits ecdh (nistp521) 0.0291s 34.4
openssl speed -elapsed eddsa
type sign verify sign/s verify/s
253 bits EdDSA (Ed25519) 0.0004s 0.0012s 2659.0 55.5
openssl speed -elapsed aes-128-cbc aes-192-cbc aes-256-cbc
# The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes 16384 bytes
aes-128 cbc 201781.99k 204240.19k 205846.50k 204732.26k 198368.08k 192202.36k
aes-192 cbc 155320.50k 165649.08k 170799.59k 174669.13k 174455.52k 176713.92k
aes-256 cbc 148357.47k 151856.76k 139485.29k 139795.69k 147752.06k 146543.36k
AES-NI CPUs:
openssl speed -elapsed -evp <cipher>
aes-128 cbc 594581.71k 704905.32k 727373.46k 734094.34k 735351.19k 728341.29k
aes-192 cbc 173568.45k 172797.10k 172932.76k 172692.18k 176029.76k 174697.20k
aes-256 cbc 151384.09k 149307.17k 153628.24k 154848.72k 154694.49k 154085.68k
aes-128-gcm 289326.68k 669727.86k 1691665.95k 2336267.43k 2923761.74k 2939784.87k
aes-192-gcm 280643.46k 789515.49k 1799382.17k 2161594.31k 2613359.92k 2599900.99k
aes-256-gcm 265146.87k 722269.03k 1588710.62k 1959085.67k 2308647.86k 2307122.45k
TLS-like benchmark:
openssl speed -elapsed -aead -evp aes-128-cbc-hmac-sha256
aes-128-cbc-hmac-sha256 25654.32k 271748.59k 1119811.51k 8298379.99k 66950407.39k 130586894.21k
openssl speed -elapsed rsa
type sign verify sign/s verify/s
rsa 512 bits 0.000057s 0.000005s 17670.1 201324.8
rsa 1024 bits 0.000152s 0.000011s 6568.0 92138.0
rsa 2048 bits 0.000726s 0.000032s 1377.9 31147.1
rsa 3072 bits 0.003061s 0.000062s 326.7 16042.9
rsa 4096 bits 0.007236s 0.000111s 138.2 9015.5
openssl speed -elapsed ecdsa
type sign verify sign/s verify/s
192 bits ecdsa (nistp192) 0.0003s 0.0003s 3022.3 3221.8
224 bits ecdsa (nistp224) 0.0005s 0.0004s 2159.8 2430.3
256 bits ecdsa (nistp256) 0.0000s 0.0001s 29636.7 9640.2
384 bits ecdsa (nistp384) 0.0011s 0.0009s 887.1 1053.4
521 bits ecdsa (nistp521) 0.0026s 0.0021s 379.8 467.0
openssl speed -elapsed ecdh
type op op/s
192 bits ecdh (nistp192) 0.0003s 3190.5
224 bits ecdh (nistp224) 0.0005s 2192.7
256 bits ecdh (nistp256) 0.0001s 12309.3
384 bits ecdh (nistp384) 0.0011s 892.2
521 bits ecdh (nistp521) 0.0025s 402.1
openssl speed -elapsed eddsa
type sign verify sign/s verify/s
253 bits EdDSA (Ed25519) 0.0000s 0.0001s 25465.9 7878.0
Do I have to upgrade my OpenVPN server with a higher spec for stronger encryption?
While at least two CPU cores are ideal, AES128 on a single core is sufficient depending on your ISP's upload speed, as it's the upload speed that determines the max throughput possible with no encryption.
Are there any server config options for stronger encryption and higher throughput?
Tuning the server and client configs provides the highest throughput possible, however it is a trial and error approach requiring time reading the OpenVPN man page and trying different tuning options to garnish the highest throughput possible (tuned config examples).
