ip sec - basic concepts

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IPSec Basics – Level-1Avadhesh Agrawal ([email protected])

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• Suite of Protocols for securing network connections

• Seen as extension to IP Protocol family

• Works pretty same way for IPv4 & IPv6

• Provides following basic services

What is IPSec?

Service Purpose

Authentication & Verification Authentication means that ensuring that data has come from authentic user only

Verification means that ensuring that data is not altered during the journey

Confidentiality Ensures that data is not visible to third party – during the journey

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• Note : These protocol are typically used independently – however can be used together (pretty uncommon) as well.

IPSec Terminology (1/2)

IPSec Protocol Purpose

Authentication Header (AH) Authenticates the data flowing over connection

Encapsulating Security Payload (ESP)

Encrypts+Authenticate the data flowing over connection

• IPSec Protocols used

• Transport vs Tunnel ModeTransport Mode Tunnel Mode

Provides a secure connection between two end-points

Provides a secure connection between two gateways or either of the end is a gateway i.e. host-to-gateway communication & vice versa

Encapsulates IP Payload only Encapsulates complete IP packet (IP Header + Payload)

Simply a secured IP connection Primarily used for VPN

Both IPSec protocols (AH as well as ESP) can function in Transport Mode

Both IPSec protocols (AH as well as ESP) can function in Transport Mode

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IPSec Terminology (2/2)

• Transport vs Tunnel Mode (Cont …)

Note : More on Transport/Tunnel mode later

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IPv4 Datagram (refresher)Field Purpose

Ver Protocol Verion (4 = IPv4)

Hlen Header Length (as count of 32bit words). Excludes payload size & other headers

TOS Type Of Service Indicator

Pkt Len Total packet length (in bytes). Including header length in bytes.

ID Used for associating fragmented packets

Flgs Mainly used during fragmentation

Frag Offset Identifies the position of this fragment in complete un-fragmented packet.

TTL Time To Live – in terms of Hops

Proto Type of protocol (UDP/TCP/etc) as encapsulated in payload. Addition for IPSec:50 : IPSec : ESP51 : IPSec : AH

Header Cksum Cheksum of entire IP Header (Payload Excluded). Not a cryptographic checksum. Purpose confined to detect errors during transmission

Src/dst IP address Source / Destination IP Address

IP Options Optional (application specific information)

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• As already discussed - through AH – authentication is done – but no - encryption

– Thus, sniffer can still read the contents of message

• Still serves three purposes

– Helps in ensuring that sender is a authentic one. In other words, we (as an receiver) are able to validate that we have received data from a valid sender only.

– Helps in detecting any alterations to data during transit

– (Optionally) prevents replaying of data i.e. malicious user reading the data & then re-injecting same data at later point in time

• Authentication is performed by computing cryptographic hash-based message authentication code

– Nearly all fields of IP packet (Header as well payload) considered

• Fields liable to change during transit are excluded – like TTL, Header Checksum

– Intermediate hops cannot (as they will not have IPSec related information for this particular connection) and need not re-calculate authentication code

AH – Overview(1/N)

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AH – Header(2/N)

Field Length (Bytes)

Purpose

Next Header 1 Protocol type of next payload. Note that, in Tunnel Mode, next payload will be IP Packet (i.e. IP Header + IP Payload (i.e. Protocol (say TCP/UDP etc) Header + Protocol Payload)s

Payload Len 1 Specifies AH Header Length. Note Don’t get confused with the field name “Payload Len”. Defined in, 32-bit words – “minus 2”. Reason for subtracting 2 is still a mystery for me.

Reserved 2 For future use – must be set to “0”

SPI 4 Security Parameter Index. In simple words, identifies security parameters associated with a given connection. More on SPI later in the ppt.

Sequence Number

4 Continuously increasing number – with every packet. Primarily to avoid replay attacks. On reaching maximum value, rather than wrapping-around – connection is re-negotiated

ICV Variable Integrity Check Value. Cryptographic hash of entire packet – however some fields are left. More on ICV later in the ppt.

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AH – Transport Mode(3/N)

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• Key noticeable points:

– Majority of fields are authenticated. Fields those can get modified during transit are skipped.

– Original IP packet is modified – as new header viz AH Header, gets added between IP Header & IP Payload.

– Note the shuffling/usage of protocol code in modified IP Packet.

• In original IP packet, proto field of IP Header was set to “TCP” where as in modified packet, proto field is set to “AH”

• Further, in modified packet, next field of AH Header is set to TCP. Helps receiver in identifying the actual protocol.

• Commonly referred as mechanism to link different headers.

– Lets see, how receiver node reconstructs the original IP packet as sent by sender node.

• Packet is Authenticated

• AH Header is removed

• Value from next (i.e. TCP) is restored in proto field of IP Header.

• Thus original packet is restored

AH – Transport Mode(4/N)

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AH

– T

unnel M

ode(5/N

)

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– Entire IP packet (IP Header + IP Payload) is encapsulated within another IP packet.

– Thus, modified packet looks like as follows:

• New IP Header (say H1) followed by

• AH header followed by

• Original entire IP packet. Note that, original IP packet remains un-modified in Tunnel Model.

– Note the shuffling/usage of protocol code

• In new IP Header, proto field of IP Header is set to “AH” – indicating that – this packet is of IPSec:AH type

• Further, next field of AH Header is set to IP. This is required because entire original IP packet is encapsulated.

– Lets see, how receiver node reconstructs the original IP packet as sent by sender node.

• Packet is Authenticated

• New IP header & AH Header is removed

• That’s it.

– Suited for VPN kind of environment where tunnel needs to be simply created

AH – Tunnel Mode(6/N)

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• How to distinguish between Transport Mode and Tunnel mode?

– Mode information not added explicitly

– Answer lies in the value stored in next field of AH header

• If next = IP then Tunnel Mode

• If next = <proto i.e. UDP/TCP/etc> then Transport Mode

• Authentication Algorithms

– SHA-1

– MD5

AH – Transport vs Tunnel Mode(7/N)

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• More complex that AH

• Allows Authentication as well as Encryption

– Authentication – Optional

• Header format is different than AH – discussed later

– Even has a trailer as well

• Provides Tunnel as Transport Mode – as in AH

• ESP Encryption algorithms commonly used

– DES, DES3, AES, Blowfish

– Algorithm & key used during encryption, is already negotiated during connection establishment phase (a new world in itself – discussed separately)

• ESP Authentication algorithms commonly used

– Same as used in AH i.e. SHA-1, MD5

ESP– Overview(1/N)

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ESP Header/Trailer(w/o Auth)(2/N)


Purpose

SPI 4 Security Parameter Index. In simple words, identifies security parameters associated with a given connection. More on SPI later in the ppt.

Sequence Number

4 Continuously increasing number – with every packet. Primarily to avoid replay attacks. On reaching maximum value, rather than wrapping-around – connection is re-negotiated

Encrypted Payload

Variable Depending upon the mode – Transport or Tunnel, Either IP payload (Transport) or entire IP Packet (Tunnel) gets encrypted here. Note: ESP Header (i.e. SPI & Sequence No.) is NOT encrypted.

Padding 0-255 Place-holder for aligning block-oriented algorithms.

Pad Len 1 Length of padding bytes

Next Header 1 Protocol type of next payload. Note that, in Tunnel Mode, next payload will be complete IP Packet.

ES

P H

ead

er

ES

P T

railer

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ESP Header/Trailer(with Auth)(3/N)


Purpose

Authentication Data

Variable Same as ICV in case of AH.

• Here, Authentication data has been added additionally.

• Only (i) ESP Header & (ii) Encrypted Payload is authenticated

• Authentication Data field in trailer – not encrypted

• Presence/Absence of Authentication is known to sender. Receiver gets to know – by virtue of SPI.

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ESP – Transport Mode (4/N)

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– Original IP packet is modified – as new header viz ESP Header, gets added between IP Header & IP Payload.

– Additionally ESP Trailer (consisting of padding, pad_len, next) gets added

– Optionally, Authentication data may also be added at the end of packet

– IP payload (i.e. TCP Header + TCP payload from previous figure) along with ESP trailer is encrypted

• ESP Header and Authentication Data (if present) – are excluded from encryption

– In case if Authentication is needed then

• ESP Header + encrypted payload + ESP Trailer - authenticated. ICV stored as Authentication Data at the end of packet

• IP Header - excluded

– Note the shuffling/usage of protocol code in modified IP Packet.

• In original IP packet, proto field of IP Header was set to “TCP” where as in modified packet, proto field is set to “ESP”

• Further, in modified packet, next field of ESP Trailer is set to TCP. Helps receiver in identifying the actual protocol.

• Note that, actual protocol type is encrypted – hence hidden from packet sniffers

ESP– Transport Mode(5/N)

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ESP – Tunnel Mode (6/N)

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– Entire IP packet (IP Header + IP Payload) is encapsulated within another IP packet.

– Thus, modified packet looks like as follows:

• New IP Header (say H1) followed by

• ESP header followed by

• Original entire IP packet. Note that, original IP packet remains un-modified in Tunnel Model.

• Then ESP Trailer followed by

• Optional Authentication Data

– Key difference from ESP – Transport Mode is

• Original IP Header (along with IP payload) gets encrypted as well.

– Note the shuffling/usage of protocol code

• In new IP Header, proto field of IP Header is set to “ESP” – indicating that – this packet is of IPSec:ESP type

• Further, next field of ESP Trailer is set to IP. This is required because entire original IP packet is encapsulated.

ESP– Tunnel Mode(7/N)

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• Security Association

• SPI

• Key Management - IKE

More To Come …

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• Pretty good tutorial for beginners

– An illustrated guide to IPSec @ unixwiz.net

Resources

ip sec - basic concepts

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