Open Medicine, Vol 2, No 2 (2008)

Analysis and comment
Email security in clinical practice: ensuring patient confidentiality
David M Kreindler
David M. Kreindler, BSc, MD, FRCP(C), is with the Division of Youth Psychiatry, Department of Psychiatry, Sunnybrook Health Sciences Centre, and the Mood and Anxiety Program, Department of Psychiatry, University of Toronto.
Competing interests: None declared.
Correspondence: Dr. David Kreindler, Department of Psychiatry, Sunnybrook Health Sciences Centre, 2075 Bayview Avenue, Room FG-62, Toronto ON M4N 3M5;

Note: Elements of this article were presented as a part of a workshop at the annual meeting of the Canadian Psychiatric Association (CPA) in Vancouver in November 2005 and at a CPA continuing medical education online forum in 2006.

Electronic mail (email) enjoys wide acceptance among the general public1,2 but has more limited acceptance among physicians.3 A 2005 survey found that 46% of Canadian physicians used email to communicate with colleagues for clinical purposes.4 Although email can be extremely useful, physicians need to appreciate the potential hazards of using email in clinical practice.5 The College of Physicians and Surgeons of Ontario (CPSO) has noted that “e-mailing or faxing unencrypted patient health information is really no more secure than sending that information on a postcard,”6 and that “those physicians who wish to send personal health information by email should use an encrypted or otherwise secure system.”7 The American Medical Informatics Association has offered similar guidance.8

This article outlines how email works, introduces the basics of encryption, and highlights the important role that encryption has to play in the clinical use of email. It does not recommend specific pieces of commercial software, since both software and encryption technology are constantly evolving.

Clinical vignette

A specialist visits a clinic in a remote city to provide consultations. While there, the physician dictates consultation notes that are transcribed by the clinic staff. The transcribed notes are sent at a later time by email to the consultant for review and editing; they are then returned to the remote clinic by email for distribution and inclusion in the patient record.

Question 1: Does the use of email in this scenario result in risks to confidentiality?

Yes, there are definite potential risks. To understand why, it is essential to first understand how email works.

How email works

Email is not sent; it is copied. In contrast to a regular letter sent via the postal service, an email message is not carried from one computer to another; rather, the original message is copied to the remote location, while the original copy remains at the sending location, in a manner similar to what happens in the transmission of a facsimile (fax).

Copies of email are generated on multiple computers. Email is relayed via a network of client and server computers, such that one copy is generated at each computer participating along the way (Fig. 1).9 This process is documented in the header of each email message and can be inspected if the header is viewed (Fig. 2).10,11 In principle, a copy of the original message could be accessed by anyone with computer expertise, physical or network access to any one of these computers, and the necessary passwords. Typically, servers retain copies of emails that they relay only temporarily, in order to conserve storage space; however, these copies are not necessarily deleted immediately. Moreover, servers are typically backed up from time to time, so there is no guarantee that copies of email in temporary storage on a server will not be preserved indefinitely as part of a backup.

Can you be sure that the sender’s and receiver’s computers are secure?. Many personal computers (PCs) are used by more than one person — for example, by family members at home or colleagues at work. Moreover, some operating systems — notably, most versions of Windows — are not configured by default to restrict one user’s ability to access other users’ files.12 Thus, it is possible that persons other than the sender or the intended recipient may be able to access email messages not intended for them on the sender’s or recipient’s computer. Also, it is possible for unauthorized users to impersonate physicians if the computer is not properly configured.

Summary. In this vignette, email is sent from clinic to consultant via mail servers on the Internet; the owners and operators of the servers are unknown. There are three loci at which someone could intercept and potentially read the clinic notes: the sender’s computer; any of the mail servers that relayed the email; and the recipient’s computer. Even if one demands that the sender and the recipient be responsible for securing access to their computers, copies of the email are generated at each of the servers; confidentiality could be breached at any one of them.

Figure 1Figure 1. Email transmission [view]
Figure 2Figure 2. A sample email message, including the header, body, and the initial portion of an attachment [view]
Question 2: What reasonable steps could be taken to decrease the risk to patient confidentiality?

In practice, there are two straightforward solutions.

  1. Strip the consultation note of all identifying patient information. However, this is typically inconvenient and time-consuming.
  2. Encrypt the email. If an email is the Internet equivalent of a postcard, the encryption process acts as an envelope.
How encryption works

Cryptography, encryption and decryption. Cryptography refers to processes for converting text from a readable to an unreadable format (“encryption”) and back again (“decryption”). Modern cryptographic systems generate a set of random digits that are then protected using a password or passphrase selected by the user, creating the key. The key is used to scramble (i.e., encrypt) the text. If viewed while encrypted, the text appears to be a string of random incomprehensible characters. Only when provided with the proper key can the original text be unscrambled (i.e., decrypted) into the original text.

Modern cryptographic systems can be divided into two broad classes,13 based on how keys are handled: secret key (or symmetric key) systems, and public key systems. These are summarized in Table 1.13-29

How secure is an encrypted document?. An encrypted document’s security — its ability to resist being read by unauthorized third parties — depends on (i) the strength of the encryption system that is being used, and (ii) the strength and confidentiality of the key. If a password protecting either the sender’s or the recipient’s key is intercepted by a third party (e.g., by being overheard, stolen, inadvertently disclosed, etc.) or the key is discovered, the document is no longer secure. Broadly speaking, there are three ways by which an unauthorized third party can decrypt and read (“crack”) an encrypted message when the sender’s or recipient’s computer is secure:

  1. Guessing the password. A thorough discussion of password selection is beyond the scope of this article.14-17 Briefly, if a cryptosystem’s key is protected by a user-supplied password, the key is vulnerable to anyone with access to the system. The degree of vulnerability depends on the strength of the user-supplied password. Words found in a dictionary are poor (“weak”) choices, since they are relatively easy to guess, as are birth dates, phone numbers, maiden names, etc., or any minor variations of these. Unusual combinations of letters, numbers, and punctuation are better choices, as are long multi-word combinations with interspersed numbers or punctuation. Long random character sequences are “strongest,” i.e., hardest to guess, but these can be difficult to remember.*
  2. Guessing the key. It is theoretically possible to decrypt an encrypted message by guessing the key that was used to encrypt it. The advantage of this method is that it does not depend on access to the sender’s or receiver’s computer. The disadvantage is that the attack — referred to as a brute force attack — is extremely resource- and time-consuming. One example of successful brute force attacks is against the widely available but poorly implemented RC-4 encryption system13,17 included with Microsoft Word 2000.18 In principal, RC-4 is a good encryption system19; however, to comply with United States export laws,17,20,21 the implementation of RC-4 in Word 2000 is limited to 240 or 1 099 511 627 776 possible passwords (technically, a “40 bit key”).22 Although this may seem like a huge number, a modern computer capable of testing 100 million potential passwords per minute can exhaust all possible alternatives in approximately 10 days. Many inexpensive software tools and services for doing exactly this are available for purchase, e.g., by searching the Internet via using keywords, “Word password decryption crack.” In contrast, one study estimated that, on average, it would take a dedicated 2001-vintage $10 million computer longer than the lifetime of the universe to break a document encrypted using symmetric encryption and a random 128-bit key.23,
  3. “Breaking” the encryption system, by finding a flaw in the math underlying the encryption system’s algorithm that can be exploited to decrypt the message. One may think of this type of attack as using an unplanned “shortcut” to a cryptosystem’s complex math problem. These kinds of shortcuts can appear in any cryptosystem, but have historically been found in cryptosystems that have been deployed with little or no peer review. One such example is the wireless encryption system used in many commercial and consumer wireless routers.
Table 1Table 1. Cryptosystem characteristics [view]

Any document encrypted with a weak key is vulnerable to guessing attacks. 17 To defend against the possibility of poor key choice, some encryption systems (such as PGP, described below) automatically generate random keys. In general, if a strong key is paired with a robust encryption system — that is, one based on Triple-DES,13,17 AES/Rijndael,25 Blowfish,17, 26, ECC,17,27 RSA,13,17 or one of the others that have survived peer review and/or public contests26 and thus can be considered reliable — the chance of unauthorized decryption is very small.

Getting started with encrypted email. In general, by default, most email programs transmit email without encryption. Although encryption is increasingly being incorporated into widely available software, no single standard for encrypted email presently exists. The closest to a single standard for public-key encrypted email, S/MIME,13,28 has been incorporated into many but not all popular email software packages, although it requires some infrastructure to work. Another popular implementation is PGP (“Pretty Good Privacy”).17,29 The following steps are key for those wishing to start using encrypted email with colleagues, patients, or others:

  1. Select cryptographic software compatible with that of your correspondents.
  2. Become proficient with your email and encryption software.
  3. Ensure that you are using strong passwords and keys, and keep them confidential.
  4. For secret key cryptosystems, decide on a shared password and communicate it to your correspondent securely.
  5. For public key cryptosystems, each correspondent will need to publish his or her public key. How this happens depends on the specific implementation chosen. S/MIME users require a digital certificate (also known as a public key certificate) from a certificate authority.17,30 A certificate authority is deemed to be implicitly trustworthy; it verifies an applicant’s identity, then issues a digital certificate. The digital certificate specifies the applicant’s email and certifies that the public key, which is included in the certificate, belongs to the applicant. Anyone wishing to send email to a user looks up the user’s public key in the certificate authority’s database online. In contrast, every PGP user generates and distributes his or her own public key. Rather than relying on the certificate authority to verify identities, PGP uses other users to vouch for the validity of each others’ public keys, thereby creating an interconnected “web of trust.”17,29 Keys and trust signatures can be hosted on PGP key servers, further simplifying this process.

*One way to remember multiple passwords is to use password management software. Password management software lets you record all your passwords in a single database, which is then encrypted with a single master password using high-grade encryption. This permits safe storage and easy access to all your passwords. Obviously, it is critical to choose a strong master password and keep it confidential.

Note that the strength of a key of a given length depends on the class of encryption system. For example, while a 128-bit key is sufficient at present for symmetric key systems, the key length that would provide equivalent security in a public key system would need to be 1620 bits in length.24


The author thanks Deborah Fenichel, PhD, and Dr. Tarek Loubani, MD, for their careful reading and detailed critique during the preparation of this manuscript.

Further reading
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